Wait what.
I'm a friendly person and would never dare to say that Eirika should go and love herself really much, so she can be happy all her life with this deserved christmas alt she got.
Galeforce has 5 CD. Even if you're doubling and Heavy Blade procs, Galeforce won't activate since your foe can't counter. You're better off swapping Galeforce for something else.
Dancers and Ostia's Pulse can help with that. Or I could have her run Moonbow instead so that she runs in, hits the enemy, gets 2 cooldown, then procs Moonbow to destroy the enemy
No? Ostia's Pulse works on all units. It's literally Pulse Tactic. It only works if you have two or less units of the same movement type (2 armor, 2 infantry, 2 cavalry, 2 flier).
With 51 attack she is not going to activating heavy blade reliably. Even with Atk Wave active, she is at 57 and misses a lot of activations. Other buffs can get you to 60+, but you shouldn't have to rely on that.
I would stick with Storm Sieglinde and Atk/Spd solo.